1. Technical Field
The present invention relates generally to local area networking of data processing devices, and more particularly to short-range data communication over twisted-pair transmission line commonly used in "Ethernet AUI cable."Specifically, the present invention relates to a dynamic threshold data receiver for receiving data transmissions over twisted-pair transmission line in such a local area network.
2. Description of the Related Art
Local area networks of data processing devices typically use coaxial cable for high-speed data transmission In a well-known configuration called an "Ethernet" local area network, a single length of coaxial cable is routed near a number of data processing stations, and the two ends of the cable are terminated with matched loads to prevent reflections. Data are transmitted over the cable at a maximum rate of 10 megabits per second by phase-shift keying a 10 MHz carrier in accordance with Manchester encoding. At each station, the coaxial cable is tapped with at least one coaxial cable interface that receives signals on the cable, and may selectively assert a signal on the cable. The coaxial cable interface is connected to the station equipment via 78 ohm, balance, twisted pair transmission line having a maximum length of 50 meters.
For duplex transmission between each coaxial cable interface and its respective station equipment, a first twisted-pair transmission line is used for transmitting 10 MHz Manchester encoded data from the station equipment to the coaxial cable interface, and a second twisted-pair transmission line is used for transmitting 10 MHz Manchester encoded data from the coaxial cable interface to the station equipment.
It is possible for "collisions" to occur between signals asserted on the coaxial cable by different coaxial transceiver interfaces. The likelihood of such collisions is reduced by a transmission protocol wherein station equipment having a message to be transmitted first checks for the presence of an existing message on the coaxial cable, and when an existing message is found, delays transmission until a predetermined duration of time after the end of the existing message. The integrity of data contained in the messages is protected from collisions by parity checks, redundancy codes, and acknowledgement protocols. To increase message throughput and decrease the likelihood of network "blocking", however, each coaxial transceiver interface checks for excessive signal levels on the cable which are characteristic of a collision. When a collision is detected, the coaxial transceiver interface sends a collision signal to its respective station equipment over a third twisted-pair transmission line. Upon receipt of the collision signal, the station equipment may immediately terminate reception or transmission of a current message, and wait for reception or begin retransmission of the current message.
In many cases, a local area network will have a multiplicity of rather autonomous data processing devices that are in close proximity to each other. Each of these data processing devices could have a respective coaxial cable transceiver. Preferably, however, the data processing devices share a single coaxial cable transceiver but are interconnected to the coaxial cable transceiver by separate twisted-pair transmission lines. In this case the coaxial cable transceiver has a single tap, but provides a multiplicity of access unit interfaces (AUIs) to the multiplicity of data processing devices Such a multi-AUI coaxial transceiver interface may substantially reduce the cost of the system, because the circuitry for multiplexing the multiple AUIs to a single coaxial cable tap and detecting local conflicts between the multiple AUIs is less complex than the circuitry for additional coaxial cable drivers, coaxial cable receivers, coaxial cable collision detectors, and hardware required for making additional coaxial cable taps.
In an "Ethernet" local area network, data transmission over the twisted-pair transmission line conforms to the Access Unit Interface (AUI) specifications of the ANSI/IEEE Standard 802.3-1988. A 78 ohm, balanced, terminated twisted-pair transmission line is driven up to a maximum length of 50 meters In an off (idle) state, the lines are maintained at a minimal differential voltage, and, at the same time, remain within a specified common-mode range. In an on (active) state, the differential voltage on the lines o rises to full-amplitude output levels, and the output amplitude is maintained for the remainder of the packet. After the last positive edge of the message packet, the transmission lines are driven at a minimum of 70% of full differential output for a minimum of 200 ns, and the differential output decays down to a minimum level for a reset (idle) condition within 8 .mu.s. Moreover, the line drivers withstand a set of fault conditions and do not suffer damage due to the faults being applied.
Line receivers for the twisted-pair AUI transmission line typically employ a "receiver squelch" circuit that allows differential receive signals to pass through as long as the input amplitude and pulse duration are greater than a minimum squelch threshold. The input amplitude limit is typically about -175 to -225 mV, and the negative-going pulses are rejected unless their pulse duration exceeds about 16 to 20 ns. The line receiver squelch typically becomes active within 50 ns after the squelch threshold is exceeded.
Integrated circuits especially designed for constructing local area networks are available from a number of manufacturers All of the transceiver electronics for a single-AUI coaxial transceiver interface have been placed on a single integrated circuit. Such a single-chip coaxial transceiver interface is sold by National Semiconductor as part No. DP8392A/NS32492A, and by Crystal Semiconductor Corporation as part No. CS83C92C. Each of these integrated circuits includes a single AUI twisted-pair transmission line receiver with a squelch circuit. For constructing multi AUI interfaces, Texas Instruments sells a dual-channel AUI driver/receiver low-power Schottky integrated circuit as part No. SN75ALS085. For use in station equipment, Advanced Micro Devices sells a part No. Am7991A/Am7992A Serial Interface Adapter including a data receiver, receiver squelch circuit, Manchester decoder, collision signal receiver, transmitter oscillator and Manchester encoder, and a data driver.
A multi-AUI coaxial transceiver interface has been manufactured and sold by Digital Equipment Corporation. The multi-AUI coaxial transceiver uses a multiplicity of integrated circuits, including a plurality of the TI SN75ALS085, interconnected on a printed wiring board. Although it would be desirable to reduce the number of integrated circuit chips in such a system, the line receivers in the TI SN75ALS085 require considerable area on an integrated circuit and consume a fair amount of power because each line receiver includes a differential data receiver and a differential squelch receiver. A logic diagram for the TI SN75ALS085, for example, is shown on page 3 of the Texas Instrument's data sheets, "SN75ALS085 LAN ACCESS UNIT INTERFACE DUAL DRIVER/RECEIVER," D3279, April 1989.